As electronic and electric devices have been miniaturized in recent years, the mainstream of coils to be attached inside such devices is changing from one using a conventional enameled wire with a circular cross section (circular enameled wire) to one using an enameled wire with a rectangular cross section (rectangular enameled wire). The rectangular enameled wire is made as a result that an insulating varnish is applied onto a conductor with a rectangular cross section (rectangular conductor) and baked, to form an insulating film. By using the rectangular enameled wire, a gap between the enameled wires when being wound into a coil can be made smaller (that is, a space factor of the enameled wire can be heightened), enabling miniaturization of the coil. Recently, in order to further miniaturize a coil, a diameter of an enameled wire is being made smaller.
For the insulating film of the enameled wire used for the coil of the motor, a resin good in a flexibility and also comparatively superior in a heat resistance, such as a polyesterimide and a polyamideimide, has been broadly used. However, the resin such as a polyesterimide and a polyamideimide, though superior in a heat resistance, is not necessarily enough since a heat resistant temperature of an enameled wire using such a resin as an insulating film material is about 200° C. Further, such a resin has a low heat deterioration resistance, and thus a fracture, a crack, a peeling from a conductor or the like sometimes occurs in the insulating film when the enameled wire is heat-deteriorated after a severe processing stress such as coiling is applied or subjected to a processing stress after being heat-deteriorated.
For such a problem, an insulated electric wire is proposed in which an insulating varnish to which an adhesion improver is added, such as high adhesion polyesterimide or highly adhesive polyamideimide, is applied to a conductor and baked, and an aromatic polyamide film is formed in an outer periphery thereof. This insulated electric wire is improved in an adhesion to a conductor of an insulating film, and a heat resistance and a heat deterioration resistance are enhanced.
However, because of formation of the aromatic polyimide film, a flexibility of the insulating film of the insulated electric wire is decreased, so that a fracture or a crack is apt to occur in the insulating film at a time of coiling. In particular, in the above-described rectangular enameled wire with a small size, a processing stress received when coiling is severer, and it is difficult to endure such a processing.